This invention concerns a secondary radar antenna working in S mode.
It is particularly applicable to S mode secondary radar systems in communication with a large number of aircraft per secondary antenna revolution.
The increase in air traffic requires that secondary radar send and receive more and more coded pulses. Within an area illuminated by antenna beams and containing an increasing number of aircraft, these coded pulses are chained to form longer and longer signals. The relative narrowness of the antenna beam, designed both for transmission and reception of coded pulses, limits the number of targets handled since the space illumination time considered necessary for transmission and reception of all these signals involved is too short.
S mode secondary radar antenna are generally single pulse and contain three channels carrying out three different antenna diagrams; the first channel is called the summing channel and is denoted .SIGMA., the second channel is called the difference channel and is denoted .DELTA., and the third channel for the suppression of secondary lobes is usually called the SLS (Side Lobe Suppression) channel. In the reception phase, the summing channel is used mainly for reception of power in signals transmitted by aircraft and therefore to be able to detect the responses contained in these signals, and the difference channel is used particularly with the summing channel to form a signal used to determine aircraft offsets from the center line of the antenna, and therefore to precisely determine the azimuth of targets.
One solution for increasing the target illumination time is to widen the summing channel, .SIGMA., antenna diagram. However since the peak transmitted power at constant average power, is low, the same is applicable for the range of the secondary radar. In particular, since the width of the main lobe of the summing channel, .SIGMA., is imposed by international standards, this parameter cannot be varied.
Also, the fast appearance of parasite network lobes when offsetting the summing diagram prevents superimposition of scanning done by the summing diagram on antenna scanning.
Another solution would be to arrange several antennas in parallel so as to increase the illuminated space without reducing the radar range, but this solution is expensive and occupies a large volume.